Steel composition

ABSTRACT

Chain parts and other steel articles are provided with hard, wear-resistant carbide coatings by tumbling them in a heated retort with a particulate mix which includes a source of vanadium and/or niobium. The steel substrate comprises a steel having at least 0.2% carbon, preferably 0.7-1.2%. Where the chromium content of the steel is 4-12%, preferably 4-8%, the chemical deposition process includes drawing a small amount of chromium from the steel substrate into the vanadium or niobium carbide coating, where it is distributed substantially homogeneously, helping to provide adhesion strength to the coating.

RELATED APPLICATIONS

This application is a division of application Ser. No. 09/891,703 filedJun. 26, 2001 now U.S. Pat. No. 6,582,765, which was based upon andclaims the full benefit of three Provisional patent applications: (1)Ser. No. 60/214,965 filed Jun. 29, 2000 bearing the title “Rotary RetortPack Method for Generating a Vanadium Carbide Layer on Small Parts,” (2)Ser. No. 60/215,050 filed Jun. 29, 2000 bearing the title “SteelComposition for Use in Making Vanadium Carbide Coated Pins,” and (3)Ser. No. 60/215,129 filed Jun. 29, 2000 bearing the title “VanadiumCarbide Coated Steel Pins for Chain and Method”.

TECHNICAL FIELD

Chemical deposition effected by a tumbling action in a sealed retort isused to form hard, wear-resistant vanadium or niobium carbide (or both)coatings on steel articles such as chain pins. Carbide coated articlesare claimed as well as a novel substrate steel and the method of formingthe coatings.

BACKGROUND OF THE INVENTION

This invention is concerned most particularly with two types ofchains—traditional roller chains and so-called silent chains. Bothroller chains and silent chains use pins as important components.

In roller chains, the pins are free to rotate in hardened bushings; insilent chains, the cylindrical pins bear directly against the insidelink apertures and are press fitted into outside guide links. The jointof the silent chain consists of the pins that rotate relative to thenon-guide row links of the chain. The silent chain uses a series oflinks that are interlaced—that is, the inside links are press fittedonto the pins; inside links in the same row of the chain, and outsidelinks in the adjacent or non-guide row, are interlaced with the links inthe links in the guide row. See the description, for example, in FIG. 4of Kozakura et al U.S. Pat. No. 6,068,568. Generally, the contact stressbetween the pin and the bushing in a roller chain will be lower thanthat of the contact stress between the pin and the link apertures of asilent chain, leading to the general observation that roller chains tendto have better wear characteristics than silent chains. This inventionis therefore aimed primarily at improving the wear characteristics ofsilent chains, but is applicable to both types of chains, and to anysteel parts, in chains or otherwise, subject to wear.

An example of pretreatment of a substrate to improve the application ofa hard coating may be found in Hale's U.S. Pat. No. 4,608,098. Achemical vapor deposition process is described in Sarin et al U.S. Pat.No. 4,957,780. FeV is employed as a carbide-forming material by Arai etal U.S. Pat. No. 4,400,224. Vanadium carbide coatings have been placedon small steel parts in the past, but generally by a salt bath proceduresuch as is disclosed in the U.S. Pat. No. 4,400,224 and/or Arai et alU.S. Pat. No. 4,778,540. See also Arai et al U.S. Pat. Nos. 4,686,117,4,786,526, 4,844,949, and 4,892,759, proposing a fluidized bed. Afluidized bed is also proposed by Lennartz in U.S. Pat. No. 5,498,442.Pins having a hard chromium carbide layer can be made by depositing thechromium from FeCr powder surrounding the pin surface, at 970 degrees C.The chromium diffuses from the powder and deposits on the pin surface,where it draws carbon from the substrate to form the carbide. Substratesteels having low carbon contents are not useful for this purpose, andaccordingly it is necessary to carburize the pins, adding to the expenseof the procedure. Nevertheless, such pins operate satisfactorily inroller chains, where the pins do not experience as much stress as thoseused in silent chains.

Chromium carbide coatings applied by chemical deposition have been triedon silent chain pins, but, under the higher surface stress of the silentchain, microbits of the hard coating spalled from the surface can add toand accelerate wear of the exposed substrate, which has a significantlylower hardness than the bits. In roller chains, the pin can wearcompletely through the bushing with the aid of loose chromium carbideparticles. The importance of the adhesion layer, which bonds the coatingto the substrate, is thus illustrated. A general observation may be madealso that good adherence of the hard coating is considerably moredifficult to achieve for vanadium carbide coatings than for chromiumcarbide coatings.

Chromium improves the adhesion of vanadium carbide coating to thesubstrate steel by forming a diffusion bond. This effect can be achievedby using ferro-chromium powder or elemental chromium powder in achromium deposition process. But the use of ferro-chromium and elementalchromium powders is frequently foreclosed or inhibited by environmentalregulation.

The composition of the pin substrate steel has significant effects onvanadium coated steel pins. We have found that appropriate carboncontent of the substrate steel can ensure the thickness of the coatingand impart strength and hardness, and appropriate chromium content inthe substrate steel is important for good adhesion of the coating to thesubstrate steel pins. Various steels having moderate chromium levelshave been disclosed in the patent literature for various purposes. See,for example, the US Patents to Sattler U.S. Pat. No. 1,773,793, CorningU.S. Pat. No. 1,496,979, Nagumo et al U.S. Pat. No. 3,907,553, Philip etal U.S. Pat. No. 3,901,690, DeSouza U.S. Pat. No. 4,224,660, Kato et alU.S. Pat. No. 4,842,818, Arata et al U.S. Pat. No. 4,902,473, Hamada etal U.S. Pat. No. 5,013,525, and Fukushima U.S. Pat. No. 5,944,920. Butno commercially available steel has been found to meet the preferredcarbon and chromium specifications. Commercially available steels havingthe desired chromium contents have a very low carbon content, requiringthat the pins and other parts or articles to be coated have to becarburized to a higher carbon level prior to the coating process, whichincreases the cost of the final products and reduces statistical qualityperformance because of the variations introduced by the carburizingprocess. Also the commercially available candidate steels tend toinclude far more molybdenum than necessary for our purposes, whichunnecessarily increases the cost of the substrate steel and the desiredarticles made from it such as chain pins. The overall cost of the coatedpin can be further reduced by lowering or eliminating this costlyelement in the steel. Our invention therefore includes a novel steelcomposition.

SUMMARY OF THE INVENTION

We have invented a process for the formation of a hard surface on steelarticles, particularly on small parts such as pins used in chains. Weemploy a rotary, or tumbling, contact diffusion process in a sealedvessel to form a vanadium carbide, niobium carbide, or mixedvanadium/niobium carbide coating on the steel article. Our rotarycontact diffusion process is a chemical deposition process carried outwith the aid of a powder mixture (pack), in which steel articles such aschain pins are immersed or mixed, containing a particulate vanadium,niobium or mixed vanadium/niobium source, preferably in the form of FeVand/or FeNb, and a halide catalyst, preferably iron chloride. When themixture in a sealed rotating or tumbling retort or other vessel isheated to the temperature range of 1600-2000° F., preferably 1700-1900°F., the catalyst reacts with the vanadium and/or niobium to producevanadium and/or niobium chloride (halide) that diffuses in the pack andis transferred, assisted by the tumbling action imparted by the rotationor rocking of the vessel, to the surface of the steel article. Thevanadium and/or niobium, as strong carbide-forming elements, drawscarbon from the substrate steel to form a carbide layer with a hardnessof over HV2000. The reactions that take place in the retort can beexpressed as: $\begin{matrix}\left. {{FeV} + {Cl}_{2}}\Leftrightarrow{{VCl}_{2} + {Fe}} \right. & {{and}\text{/}{or}} & \left. {{FeNb} + {Cl}_{2}}\leftarrow\left. \rightarrow{{NbCl}_{2} + {Nb}} \right. \right. \\\left. {{VCl}_{2} + C}\Leftrightarrow\left. {VC}\downarrow{+ {Cl}_{2}} \right. \right. & \quad & \left. {{NbCl}_{2} + C}\leftarrow\left. \rightarrow{{NbC} + {Cl}_{2}} \right. \right.\end{matrix}$

Succinctly, our invention involves depositing vanadium and/or niobium onthe surface of the steel article, in the form of a halide, by chemicaldeposition effected by tumbling. The halide is transformed to vanadiumand/or niobium carbide on the surface of the steel article, the carbonfor displacing the halide and combining with the vanadium and/or niobiumbeing diffused from the steel substrate. Accordingly, we prefer thecarbon content of the steel be in the range of 0.7% to 1.2% by weight.

We also prefer a chromium content in the substrate steel of 4% to 8% byweight. Chromium improves the adhesion of the vanadium or niobiumcarbide coating to the substrate steel by forming a diffusion bond. Buttoo much chromium in the substrate steel may promote ferrite stabilityand tend to inhibit the formation of a fully martensitic structureduring post-heat treating. Accordingly, we use chromium contents in thesteel lower than those of many stainless steels. More generally, we havefound that chromium contents in the range of 4% to 12% may be used inour process, although 4-8% is preferred, and 5-6% is most preferred.Chromium contents higher than 12% or lower than 4% may be used where theadhesion bonding is not particularly useful.

We have found that, not only does carbon migrate to displace the halidefrom newly deposited vanadium or niobium halide, but chromium alsomigrates from the substrate steel to the coating, forming chromiumcarbide, which is distributed throughout the vanadium and/or niobiumcarbide coating. This substantially homogeneous dispersion of chromiumcarbide throughout the vanadium/niobium carbide coating is unique in ourexperience and contrasts to the less desirable substantially stratifiedresult when a source of chromium is mixed with a vanadium or niobiumsource in the powder pack.

Vanadium and niobium are in Group 5 of the Periodic Table of theElements in the 18-group classification designated and recommended bythe International Union of Pure and Applied Chemistry. Since niobium hasan atomic number of 41, we intend for the phrase “Group 5 metal havingan atomic number no greater than 41” as used herein to mean vanadium andniobium.

Accordingly, our invention includes a method of forming a hard coatingon a steel substrate having a composition including 0.7% to 1.2% carbonand from 4% to 12% chromium comprising applying at least one Group 5metal having an atomic number no greater than 41 to the surface of thesteel substrate by chemical deposition effected by tumbling at atemperature of 1600° F. to 2000° F. The Group 5 metal source ispreferably FeV or FeNb. In addition to the Group 5 metal source, achromium source may also be used in the chemical deposition process, butthe amount should be such that not more than 5% of the resultant metalcarbide coating is chromium carbide originating from the particulatemix.

Other, substantially inert, particulates, such as aluminum oxide, may beincluded in the particulate mix, preferably in amounts no greater than50% by weight of the total mix.

In the chemical deposition process, a halide catalyst is used. We preferto use a chloride, and particularly FeCl₃, as the catalyst, but ammoniumchloride, niobium chloride, and vanadium chloride are also particularlyuseful forms. A small amount of halide from the catalyst is believed tocombine temporarily with the Group 5 metal (and the chromium metal, ifany is present in the particulate) and is released at the articlesurface when it is displaced by carbon drawn from the steel article. Thehalide is therefore arguably an activator rather than a catalyst. We donot intend to be bound by any one of such distinctions in the theory ofthe effect of the halide, but have adopted the term catalyst as a matterof convenience and employ it throughout to imply either function of thehalide or any other theory of its function.

In a more preferred aspect, our invention includes a method of making awear-resistant steel article comprising forming the article from steelhaving a carbon content of 0.9% to 1.1% by weight and a chromium contentof 4% to 8%, most preferably 5-6%, by weight, and forming Group 5 metalcarbide on the surface of the article by chemical deposition of theGroup 5 metal thereon. In our preferred method of chemical deposition,the incipient wear-resistant steel article is tumbled in a vessel,preferably by rotation of the vessel, containing a particulate mix of asource of Group 5 metal, preferably vanadium, and, after a period of atleast 60 minutes at a temperature of 1600-2000° F., preferably1700-1900° F., the article is gradually cooled and separated from theparticulate mix.

While we may use a variety of steels having carbon and chromium contentsas stated above, our most preferred substrate is a steel having acomposition, by weight, as follows: carbon: 0.9 to 1.1%; chromium: 5 to6%; manganese: 0.25-0.45; silicon: 0.25 to 0.55%; molybdenum 0.2 to0.55%; phosphorous 0.03% maximum; sulfur: 0.03% maximum, and aluminum:0.05% maximum. More generally, our preferred steel is

C Cr Mn Si Mo P S Al 0.7-1.2 4-8 0.25-0.45 0.25-0.55 0.2-0.55 0.03 max0.03 max 0.05 max

The balance comprises iron and may include small amounts of otherelements and metals commonly used or found in steel. This steelcomposition is called hereafter Steel Composition A.

After the carbide coating is formed, the coated articles have a novelstructure comprising a coating and a core, the coating comprisingvanadium carbide and/or niobium carbide, the core comprising SteelComposition A. In addition to the vanadium and/or niobium carbide, wherethe steel substrate contains at least 4% chromium by weight, the coatingcomposition includes 1% to 3% percent chromium carbide distributedsubstantially homogeneously throughout.

Our invention further includes a post-heat treatment wherein, aftercooling and separation from the particulate mix, the coated article issubjected to at least austenitizing temperature and quenched in aconventional manner to harden the core, preferably achieving a finalcore hardness Rc44-56; the article may then be polished in aconventional manner. However, the coated article is novel without thepost-heat hardening process, and is useful, for example, for thecorrosion resistance imparted by the coating.

In another aspect, our invention includes a method of forming a hardcoating on a steel article. The method utilizes contact of the article,preferably by tumbling the article, with a particulate source of Group 5metal having an atomic number no greater than 41 in a heated vessel inthe presence of a halide catalyst. The tumbling may be accomplished byrotating the vessel and its contents. The steel should have a carboncontent of at least 0.2% by weight. The article is preferably mixed inthe particulate source of Group 5 metal and catalyst, and the vessel isrotated to mix its contents and provide more or less continuous contactof the article with the particulate materials while the temperaturewithin the vessel is maintained at 1600-2000° F. (870-1200° C.),preferably 1700-1900° F., for a time sufficient to form a coating ofGroup 5 metal carbide of the desired thickness on the article, having ahardness of at least HV2000.

Preferably, air is withdrawn and the process is conducted in the sealedvessel in the substantial absence of air, but we prefer to fill the voidwith an inert gas, preferably argon or nitrogen, rather than use avacuum, which requires greater vigilance against leaks. During theheating and rotation of the vessel, the ferrovanadium and/orferroniobium, or other source of Group 5 metal, is caused to dissociate,providing Group 5 metal which is deposited at the surface of the articlein the form of a halide. Carbon is drawn from the steel surface of thearticle to displace the halide, which then reverts to the particulatemix to combine with additional group 5 metal from the source. Only asmall percentage of the Group 5 metal source, estimated at 0.5 to 2%, ofthe metal in the metal source, is consumed in the process to provide acommonly desired coating thickness of 10 to 20 microns.

Our method invention includes two or more uses of the particulate mix.That is, after the article or articles are treated to form a hardcoating as described above, the particulate mix and the aritcles areseparated, and the particulate mix may be returned for re-use in thevessel (or a different one) to be heated again in the presence ofanother article or articles to be coated. The particulate mix need notbe replenished through several iterations, but our invention includesthe possibility of replenishing the Group 5 metal source and/or thecatalyst while the bulk (at least 50%) of the particulate mix insuccessive uses may comprise material having been used before for thepurpose. Since generally less than 2% of the vanadium source is consumedin a single use, and since the halide displaced from the Group 5 metalat the surface returns to the particulate mix to combine with additionalGroup 5 metal, our invention includes the use of the same batch ofparticulates for at least two batches of articles, and additionalbatches as the economics of the facility may suggest. Generally at leastfive uses will be quite practical. Preferably, for any given use, theratio of Group 5 metal in the Group 5 metal source to the articles willnot be below 1:2 by weight, and is preferably 1:1 to 2:1 by weight.There is no theoretical reason why the ratio cannot be considerablyhigher than 2:1—say, 10:1, but such a ratio would not normally beeconomically acceptable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a longitudinal sectional view of a preferred rotating retortcontaining a vanadium source and prepared for forming a coating onselected articles. FIG. 1B is an end section of the rotating retort alsoshowing the contents.

FIG. 2 is an idealized section of a pin coated with vanadium carbide byour process.

FIG. 3 is an idealized plot of chromium carbide against depth from thesurface of a pin coated by our process.

FIG. 4 shows a portion of a silent chain generally of a prior art designbut including pins which may be coated by our process.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1A, the method is preferably implemented in arotary container 1 having a shaft 9 held rotatably in walls 2 and 3 offurnace 4 by bushings 7 and sealed. A motor not shown will rotate thecontainer 1 at a desired speed while the furnace 4 is maintained at atemperature of 1600-2000° F., preferably 1700-1900° F. Inside container1 is a particulate mix 5 and at least one steel article, in this casesteel chain pins 6, to be coated. In this exemplary illustration, theparticulate mix 5 comprises ferrovanadium having a particle size of 0.8to 3 mm, and includes about 1% of the selected halide catalyst, FeCl₃.FIG. 1B is an end section of the container, illustrating how thecontents are mixed, preferably with the aid of baffles 8, duringrotation of the container 1. The particulate mix and the article(s) tobe coated are substantially constantly contacted during the rotation ofthe container 1, causing the vanadium carbide to be formed on thesurface of the steel chain pins 6.

The halide catalyst may be used in any effective amount, but we preferabout 0.6% to 3% by weight of the Group 5 metal source. The vessel,retort, or container 1 may be rocked or otherwise agitated rather thanrotated.

Our invention includes a steel article coated with a carbide of a Group5 metal having an atomic number no greater than 41. A preferred articlewill have a core of steel comprising 0.8-1.2% carbon (most preferably0.9-1.1%) and 4%-8% chromium (most preferably 5-6%) and a hard coatingcomprising vanadium carbide, most preferably vanadium carbide including1% to 3% chromium carbide distributed homogeneously throughout thecoating. All percentages herein are by weight.

In FIG. 2, a silent chain pin finished by our method is shown insection. The pin comprises core 10 and a hard outer coating 11. The core10 is steel of Steel Composition A, and the outer coating 11 is acoating comprising 97-99% vanadium and/or niobium carbide and 1-3%chromium carbide distributed homogeneously throughout. During thecoating process, the Group 5 metal is deposited on the article surfacein the form of a halide, and carbon is drawn from the surface of thesteel to combine with the Group 5 metal and displace the halide.However, it is not essential for our process to use the preferred steelsubstrate having the above prescribed amounts of carbon and chromium.Our tumbling contact process may be used beneficially on any steelhaving at least 0.2% and up to 2% carbon by weight.

FIG. 3 is an idealized plot of chromium carbide content against coatingdepth for a coating 15 microns thick—that is, distance from the coatingsurface on a pin treated by our process. In this typical case, the steelhad a carbon concentration of 1.0% and a chromium concentration of 5.5%.It will be seen that there is a gradient of chromium carbidehomogeneously dispersed and ranging in concentration from about 1% nearthe surface of the coating to about 3% in the lower region of thecoating. As discussed elsewhere herein, the chromium carbide is formedfrom chromium and carbon present in the steel prior to the use of ourprocess.

In FIG. 4, a portion of a typical silent chain is shown, comprising setsof plates A and B, each having two holes for pins 20. In thisconfiguration, parallel sets A of four plates and parallel sets B ofthree plates may be shaped to accommodate sprockets or otherwise toengage a force-delivering device not shown. Some of the plates A or Bmay articulate on the pins 20 and others may be secured to them so asnot to rotate on the pins, depending on the design of the chain. Ineither event, whether there is articulation or not at the plate/pininterface, significant stress and wear may be engendered at theinterface of the pins and the plates.

A comparison of chain pins made by our process to more conventional pinsshowed that the hard coating on our pins did not flake off the pin whenit was bent in a vise, whereas pins made by a conventional processflaked off. This is generally taken to mean that when the coating of ourpin is abraded, it will nevertheless adhere more tenaciously than thecoating of the conventional pin. As indicated above, flaking or spallingof hard coatings can be very destructive to worn contact surfaces ofchain parts.

What is claimed is:
 1. A layered steel composition comprising, in weightpercent, (a) a core composition comprising: C Cr Mn Si Mo P S Al 0.7-1.24-8 0.25- 0.25- 0.2- 0.03 max 0.03 max 0.05 max 0.45 0.55 0.55

and (b) a coating composition thereon comprising 97-99% by weightcarbide of at least one Group 5 metal having an atomic number no greaterthan
 41. 2. A layered steel composition of claim 1 wherein said coatingcomposition includes 1-3% by weight chromium carbide.
 3. A layered steelcomposition of claim 1 wherein said Group 5 metal comprises vanadium. 4.A layered steel composition of claim 1 wherein said Group 5 metalcomprises niobium.